Human replication protein A (hRPA) is an essential single-stranded DNA-binding protein which stimulates the activities of DNA replication and repair proteins by physical interaction with them. It exists as a stable heterotrimer of 70, 29, and 11 kD subunits. There are at least two binding modes for hRPA and ssDNA, hRPA8nt and hRPA30nt. The STEM was used to show that the less stable hRPA8nt complex was quite compact, whereas the elongated hRPA30nt complex exists in both contracted and extended forms. Formation of the extended hRPA30nt complex correlates with increased phosphorylation of the hRPA 29kD subunit by DNA-dependent protein kinase.